bdb: expose more db types.

[nodech]

This allows db consumers to use Batch/Iterator/etc. to use these types as args in typedefs.

e.g.

/** @typedef {import('bdb').DB.Batch} Batch */

class AbstractMigration {
  ....
  /**
   * Run the actual migration
   * @param {Batch} batch
   * @returns {Promise}
   */

  async migrate(batch) {
    throw new Error('Abstract method.');
  }
}

[nodech]

Alternative is here: pinheadmz/bsdoc@74ab5ef

This can be used to use typescripts: ReturnType helper instead and grab Batch etc like:

/** @typedef {ReturnType<import('bdb').DB['batch']>} Batch */